Priority Interrupt

Firstly let us discuss a brief about the history of this topic.Priorityinterruptis one of the methods of datatransferfromCPUto peripheraldevices. It is in actual a sub part of the main method i.e.interrupts initiated. Data istransferred fromCPUto I/Odevices on the initiation ofCPU. But, theCPUcannot start thetransferunless thedeviceis completely ready for communication with theCPU. The readiness of thedeviceis checked only by theinterruptgiven by thedevice. TheCPUresponds to the request by storing the returned address from PC into thememorystack. Then the next job is of program to service that request bytransferof data.

In a typical system number of I/Odevices are connected to the system. Each of thedevices is equally capable of producing theinterruptto talk toCPUandtransfers the data. The first task of theinterrupthandler is to identify the source of the request and the time it originates. Sometime it also happens that the requests generated are simultaneous. In such cases system has to decide which one to be served first.

Priorityinterruptis a system which assignspriorityto the variousdevices and arranges them in order to serve them in case of simultaneous requests. The system may also decide determine which conditions are permitted tointerruptthe computer while another request is being serviced or in other words it is responsible for determining theinterrupting scope of thedevice.

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Accordingly we can easily judge which task need to be of higherpriority. Tasks which can’t be delayed orinterrupted while execution otherwise some serious consequences can happen are always prioritized higher. For exampledevices like magnetic tapes which have hightransferrate are prioritized higher then thedevices with slow datatransferlike keyboard. As simple as it can be, the computer will serve that request first who’s priorityis higher then the other if there is any conflict between the request creations.

Establishing thepriorityof theinterruptcan be through software or hardware. A polling process is used to identify the highestprioritysource by means of software. In this method there is one common branch address for allinterrupts. The programs that care about theinterruptbegin from the branch address and poll theinterruptin sequence. The order in which they are tested determines thepriorityof each source in sequence. The highestpriorityis served first and then the count down starts.

The major disadvantage of this method is that if too manyinterrupts are generated and needed to be prioritized then the time required to prioritize them exceeds the time available for the service of I/Odevice.

In hardwarepriorityinterrupthandler, there is one hardware unit that receives theinterruptfrom all thedevices and then decides theprioritywithout polling and is thus faster and efficient.

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